TTL slave strobe with a pre-flash camera

I wonder how the Ikelite strobes with a TTL slave sensor

(DS-50,125) can possibly work correctly with pre-flash

cameras (that is, use the camera's TTL metering)? The

camera fires a pre-flash (triggering the external strobe's

pre-flash), meters the result and calculates the

illumination needed for the real exposure, under the

assumption that the pre-flash it just saw was the result of

its own strobe with the intensity it knows. It then sets

not only the duration of the main flash, but also its

intensity (amount of light = duration x intensity). What

happens then -- the external strobe's sensor can sense the

duration of the main internal flash, but surely it cannot

determine its intensity, right?

Or is it known that IN MOST CASES, the camera changes only

the duration of the internal flash not the intensity, that

is, the pre-flash and the main flash have the same

intensity (and it's only in those cases that this TTL

sensor setup works)?

I've read here that, for example, Canon G2 plus DS50/125 in

TTL mode seem to work, just wonder how it can possibly be

so.

Thanks

mmm

In the Olympus Cx0x0 family the camera uses the pre-flash to calculate the duration of the main flash. When the camera thinks that the correct exposure has been reached it quenches the internal flash.

The DS125/DS50 with the TTL slave sensor mimmicks this sequence. The DS125 also has a pre-flash switch which allows you to set a longer or shorter pre-flash duration of the DS125.

See the oscilloscope images here re: flash duration. Notice the height of the pulses are consistent:

http://www.muenster.de/~matthias/blitz/indexe.htm

Sorry, mmm, but there is a mistake in what you said ;-)

You said : amount of light = duration x intensity... and you are right !!

But when you said "the camera cannot determine the external strobe intensity" ... you are wrong !

Here is the explanation : when the external strobe fires the pre-flash, the computer knows the duration of this pre-flash and can assess the amount of light, right?... then it can calculate the external strobe intensity (amount of light / duration).

Then, assuming that the intensity does not depend on the duration (which is almost true!),

the camera can calculate the necessary duration to reach the correct amount of light... and triggers the second and main flash with these data!

Regards!

Put more simply:

The camera adjusts only the flash duration. (The firing capaciters in typical strobes only discharge at one speed ... full.) If the External strobe matches the duration of the built-in strobe, that's all that's required.

-Mickey

To Benoit:

> But when you said "the camera cannot determine

> the external strobe intensity" ...

I meant to say that the external strobe's sensor

cannot determine the intensity of the internal

strobe (should have worded it more clearly).

To ReyeR and Mickey:

Yes, I now think you're right. I just re-read

the source where I got this idea that the flash

power is adjusted along with the pulse duration

(http://photonotes.org/articles/eos-flash) and

figured I've been misled by its sometimes loose

use of the term "power".

So those manual GN switches on the external

strobes actually change the pulse duration, too?

mmm